Oxygen sensing probes are well known conventional instruments which sense the oxygen content of a gas. In the industrial furnace field, the oxygen is correlated by known formula to CO.sub.2 or dew point (water vapor) composition of the gas which is used to control the furnace atmosphere and thus the heat treat process. In the industrial heat treat art, the oxygen sensing probe is usually of the type which has a solid electrolyte which is in contact on one side with a reference gas (air) and on its opposite side with the furnace atmosphere whose oxygen content is to be measured. When the electrolyte is stabilized zirconia, an Emf is generated between the sides of the electrolyte. The magnitude of the voltage generated from the ion activity of the electrolyte is dependent upon the temperature of the electrolyte and the log of the ratio of the oxygen partial pressure on the opposing sides of the electrolyte. Another type of oxygen sensor, used in the automotive field, is one wherein the electrical resistance of the sensor, i.e. titania, changes with the amount of the oxygen present in the gas. The present invention is directed in its preferred embodiment to the oxygen sensing probe which uses a stabilized zirconia electrolyte although the invention is believed applicable to a titania sensor.
It is well known in the industrial furnace art to use oxygen sensing probes to control the furnace atmosphere. The probes are conventionally used for atmospheres ranging anywhere from neutral (i.e. typically used in furnace annealing heat treat processes) to rich or reducing or endothermic (i.e. typically used in carburizing heat treat furnaces). However, there is theoretically no reason why the oxygen sensing probe cannot be used to sense lean or oxygen enriched furnace atmospheres so long as the oxygen content in the furnace atmosphere is less than the oxygen content present in air.
When the oxygen sensing probes are used in a rich or reducing furnace atmosphere heavy in hydrocarbon concentration (such as carburizing processes which require the carbon potential in the atmosphere to be closely controlled), carbon deposits accumulate on the probe and in time adversely affect the probe's reading. The life of the probe in such applications is significantly reduced. The same problem, short life, is encountered in endothermic gas generators which generate rich, reducing gases typically supplied to carburizing furnaces.